Such materials are characterized by their pore volume, their specific surface area and their X ray diffraction spectrum. These materials have a high adsorbent capacity, and are usually used as catalyst supports, adsorbents, or separation means.
The preparation of oxides with a controlled porosity is based on the fact that organic molecules which are classified as surfactants can associate together, in particular when they are placed under controlled porosity oxide synthesis conditions. They then form micelles which have surface electrical charges and a variety of geometries depending on the conditions of the synthesis medium.
Such micelles can then structure polymeric species of mineral oxides such as silica, alumina or other metals such as titanium about them, and thus generate a solid mesoporous network.
International patent WO 96/39357 describes a complementary route using non-ionic surfactants such as polyethylene oxides and neutral precursors of inorganic oxides as sources of the reactants. Such a synthesis is carried out in an organic solvent. This method enables the surfactant to be eliminated readily. Porous inorganic oxides are prepared from non-ionic surfactants using that method.
Studies primarily aimed at controlled hydrolysis of organic Al(OR').sub.3 type aluminium precursors, where R' is a linear or branched alkyl group containing 1 to 5 carbon atoms, highlighted the case where the structuring agent is a quaternary ammonium type, where the medium is a mixed water-alcohol type, and the case where the structuring agent is a micellar type, the reaction then being carried out in an organic solvent. Vaudry and Davis in Chem. Mater., 1996, p. 1451 used fatty acid type surfactants. S. Bagshaw, T. J. Pinnavaia, in Mesoporous alumina molecular sieves, Angew. Chem. Intl. Ed. Engl. 1996, 35; 10, p. 1102-1105 described the production of lamellar phases.
When the structuring agent is a quaternary ammonium type R.sub.4 N.sup.+, the X ray diffraction diagram generally exhibits a single diffuse diffraction below 2.theta.=5.degree. (CuK.alpha.), indicating the presence of a mesostructure with a periodic short range order. With micellar surfactants, the X ray diffraction diagram generally exhibits a peak corresponding to an interplanar distance which is strictly over 3 nanometers indicating a periodic arrangement with a longer range order.
A mesoporous alumina with a hexagonal structure has been synthesised from Al.sup.3+ species in the presence of sodium dodecyl sulphate; that synthesis has been described by M. Yada, M. Machida, T. Kijima in: "Synthesis and deorganization of an aluminium-based dodecyl sulphate mesophase with a hexagonal structure", Chem. Commun., 1996, 769-770. That method requires the use of large quantities of urea the decomposition of which leads to a slow increase in the pH during prolonged heating. Thus the Al.sup.3+ cations are hydrolysed, which enables them to polycondense to a lamellar phase then to a hexagonal phase when the pH is above 7. That structure is preserved after calcining the unwashed product but no mention was made of the existence of controlled porosity.
A. S. Stein, B. Holland, in "Aluminium-containing mesostructural materials", J. Porous Materials, 1996, 3, 83-92 synthesised an alumina from Keggin type polycations based on Al.sub.13 aluminium in the presence of dodecyl sulphate. However, after heat treatment to eliminate the surfactant, the solid obtained had a very low specific surface area of 18 m.sup.2 /g, obtained using the BET method.
In the present description, the abbreviation "nm" will be used for "nanometer", i.e., 10.sup.-9 meters.